In a refrigerating cycle that can be used for freezing or air conditioning, if a pressurization or pressure reduction process is performed by a scroll-type fluid machine, appropriately supporting or handling gas load that acts axially on an orbiting scroll as a result of differential pressure between an inlet and an outlet, also known as “thrust load”, is important for increasing cycle efficiency and ensuring reliability.
In refrigerating cycles such as those that use carbon dioxide as a refrigerant, in particular, high-to-low differential pressure is extremely large. For this reason, supporting thrust load is difficult by conventional methods such as supporting the thrust load by a hydrodynamically lubricated surface.
In order to solve problems of this kind that relate to supporting thrust load, double-sided scroll fluid machines are commonly-known in which spirals are disposed on two surfaces of a base plate of an orbiting scroll, the two spirals of the orbiting scroll are mated with respective spirals of fixed scrolls that are disposed on two sides of the orbiting scroll such that a compression chamber and an expansion chamber are formed on both sides of the orbiting scroll, and axial thrust loads that act on the orbiting scroll in the compression/expansion process are canceled out.
In these double-sided scroll fluid machines, spirals are formed on two surfaces of a base plate of an orbiting scroll, and a main shaft that drives or supports the orbiting scroll is supported at two ends by shaft bearing portions that are disposed centrally on the two fixed scrolls so as to pass through central portions of the spirals of the orbiting scroll. Here, it is necessary for ports that are formed on bottom surfaces of winding start portions of the spirals of the fixed scrolls to be positioned outside an orbiting motion range of a boss portion of the orbiting scroll through which the main shaft passes. Thus, in order to ensure port aperture area, it is necessary for the winding start portions of the spirals of the fixed scrolls to be positioned partway along involute curves near outer circumferences of the shaft bearing portions, reducing efficiency. If attempts are made to ensure port aperture area by disposing the winding start portions of the spirals of the fixed scrolls closer to the starting points of the involute curves, the port openings interfere with the boss portion of the orbiting scroll, increasing fluctuations in the port aperture area.
To solve problems of this kind in double-sided scroll fluid machines, double-sided scroll fluid machines have been proposed in which a peripheral wall surface near a scroll center of a spiral groove of a fixed scroll is formed into a semi-circular surface, a port opening is disposed on the semi-circular surface on an inner wall side of the spiral groove, an inner peripheral end of a spiral lap of an orbiting scroll slides in contact along the peripheral wall surface near the scroll center, and a working fluid is discharged through or sucked into the port opening (see Patent Literature 1, for example).
In conventional scroll fluid machines such as that described in Patent Literature 1, because the port opening is formed on the peripheral wall surface near the scroll center of the spiral groove of the fixed scroll, interference between the port opening and the boss portion of the orbiting scroll can be avoided. Thus, the winding start portions of the spirals of the fixed scrolls can be shifted toward a starting end of the involute curve compared to when a port opening is formed on a bottom surface of the spiral groove, enabling increased efficiency.
Although not double-sided, scroll compressors that have a penetrating axis construction have been proposed in which two discharge ports are disposed on a fixed scroll, and these discharge ports are disposed symmetrically about a central axis of the fixed scroll (see Patent Literature 2, for example).
In conventional scroll compressors such as that described in Patent Literature 2, vibration and noise are reduced by making a discharging process balanced, enabling effects that can improve reliability.    Patent Literature 1: Japanese Patent Laid-Open No. HEI 11-141301 (Gazette)    Patent Literature 2: Japanese Patent Laid-Open No. HEI 04-234591 (Gazette)